Insert foreign key to create User

Good afternoon everyone. I am creating a form to MANAGER , which filled automatically creates a user (subscriber ) with those data. But I need time to create a user , the Manager ID, ​​go to foreign key wp- user table. I can do most , however the foreign key does not receive the value Manager ID , is to 0. It sure is why I created this separate field , then it is not a standard field that wp- user table. My code below: